Interrelationships of trace metals in rat and mouse tissues are being studied as to effects of 15 abnormal metals fed for life on tissue concentrations of chromium, manganese, copper and zinc. Effects of 12 trace elements on the fertility and viability of offspring of breeding rats and mice constantly exposed to the metal from the time of weaning are being studied. Life-term studies of rats exposed to low doses of 5 trace elements are nearly completed, and those of mice are finished.